KR101299803B1 - Method for manufacturing low-alloy high-strength cold rolled thin steel sheet with excellent weldability - Google Patents

Abstract

The present invention uses the thin slab playing method and by using the hot-rolled controlled cooling and cold rolling and annealing process to contain 90% or more of the ferrite as the final microstructure without installing a quenching equipment such as water cooling, weldability, yield ratio, elongation, The main object of the present invention is to provide a method for producing a high strength cold rolled steel sheet having a yield strength of 750 MPa or more, which is excellent in all required properties such as bending workability.
The production method of the present invention for achieving the above object, by weight% C: 0.01 ~ 0.12%, Si: 0.1 ~ 2.0% or less, Mn: 0.5 ~ 2.5%, P: 0.001 ~ 0.10%, S: 0.020% Al: 0.02 to 0.30%, N: 0.020% or less, Cr: 0.1 to 1.2%, B: 0.0010 to 0.0080%, tramp element (Cu + Ni + Sn + Pb): 0.18% or less, Ti: 0.003 ~ 0.08%, Nb: 0.003 ~ 0.08% of one or more are added, continuous casting of steel consisting of the remaining Fe and other unavoidable impurities into a thin slab with a thickness of 30 to 150mm, and rough rolling, heating, finishing In the method of manufacturing a high-strength cold-rolled steel sheet through a rolling and winding step, and the hot rolled strip is pickled, cold rolled, continuous annealing and cold heat treatment step, the C, Si, Mn, Cr, P , S is configured to satisfy [C + Mn / 6 + Cr / 5 ≤ 0.55] and [C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.27], wherein the winding step comprises the finished rolled strip 20 ℃ / s or more cold After cooling at a rate, the coiling temperature (CT) is configured to be wound in a range satisfying the [555 -440C - 34Cr 14Si - 26Mn - 11Cr ≤ CT ≤ 600 - 58C - - 35Mn].

Description

TECHNICAL FOR MANUFACTURING LOW-ALLOY HIGH-STRENGTH COLD ROLLED THIN STEEL SHEET WITH EXCELLENT WELDABILITY

The present invention relates to a method for manufacturing a low-alloy high strength cold rolled steel sheet excellent in weldability, and more particularly, by using a thin slab continuous casting method, it is possible to improve weldability, bending workability, etc. with a small amount of alloying elements without a quenching facility. It relates to a method for producing an alloy high strength cold rolled steel sheet.

Recently, in the automotive industry, steel sheets having higher strength are required to improve fuel efficiency and durability by various environmental regulations and energy use regulations. In particular, as the impact safety regulations of automobiles have spread, high-strength steel having excellent yield strength has been adopted for structural members such as members, seat rails, and pillars to improve impact resistance of the vehicle body. The structural member has a characteristic that the higher the yield strength than the tensile strength, that is, the higher the yield ratio (tensile strength / yield strength), the better the impact energy absorption capacity. However, in general, when the strength of the steel sheet is increased, the elongation is reduced to reduce the formability, so the development of a material that compensates for this is required.

Typically, the method of strengthening steel is divided into solid solution strengthening, precipitation strengthening, strengthening by grain refinement, and transformation strengthening. Among the above methods, the strengthening by solid solution strengthening and grain refinement has a problem that it is very difficult to produce high strength steel having a tensile strength of 490 MPa or more.

Precipitation hardening type high strength steel has the effect of grain refinement caused by the addition of carbon and nitride forming elements such as Cu, Nb, Ti, V, etc. Strength is secured. Therefore, there is an advantage that high strength can be easily obtained in comparison with low manufacturing cost. However, since the recrystallization temperature is rapidly increased by the fine precipitates, there is a problem that high temperature annealing must be performed in order to cause sufficient recrystallization and ensure ductility. In addition, precipitation-reinforced steel, which precipitates and strengthens carbon and nitride on ferrite base, has a problem that is difficult to manufacture high strength steel of 600MPa or more.

Representative prior arts for precipitation hardened high strength steels include Japanese Patent Laid-Open Nos. 56-84422 and 4-221015.

Japanese Patent Application Laid-Open No. 56-84422 contains one, two or more kinds of Ti, Nb, V, and the like based on low carbon steel containing 0.15% or less of C, and has a finish rolling temperature of 750 to 950 캜 and a coiling temperature. It is limited to 450 ℃ or less. However, this conventional technique is effective in improving the strength by forming extremely fine precipitates because the coiling temperature is too low, but it does not secure the strength of the yield strength of more than 750MPa, but also causes the shape defect of the hot rolled steel sheet and increases the residual stress around the precipitate. Therefore, there is a problem that an overload phenomenon occurs during cold rolling.

Japanese Patent Laid-Open No. 4-221015 proposes a precipitation strengthening type high strength steel which can increase the strength by accelerated cooling after hot rolling using Nb or V as a precipitate forming element. However, even in this case, the winding temperature is set to 400 ° C. or less, so that not only has the same problems as in the prior art, but also bainite or martensite structure is formed instead of forming a uniform ferrite structure, resulting in low yield ratio. Have

On the other hand, the transformation hardened high-strength steel is a ferritic-martensitic dual phase steel in which a hard martensite is included in a ferrite matrix, or a transformation induced plasticity (TRIP) steel sheet using transformation organic plasticity of residual austenite, and ferrite and hard steel. Various composite steels have been developed, such as bainite or martensite, and complexed phase (CP) steels composed of carbides.However, a large amount of Mn, Si is formed to form hard or residual austenite to cause transformation strengthening. Alloying elements, such as these, should be added. However, such a hardenable element has a disadvantage in that the weldability is impaired and the plating property is reduced by concentrating Mn and Si-based oxides on the surface during annealing.

BACKGROUND OF THE INVENTION A method for manufacturing a transformation hardened high strength steel is disclosed in many prior arts. As an example, Japanese Patent Laid-Open No. Hei 6-145892 discloses a method for producing a steel sheet having excellent press formability by controlling a specific chemical composition and the residual austenite content of the steel sheet. Japanese Patent Nos. 2660644 and 2704350 disclose a method for producing a steel sheet having good press formability by controlling a specific chemical composition and the microstructure of the steel sheet. In addition, Japanese Patent No. 3317303 discloses a method for producing a steel sheet having excellent workability, particularly local stretching, including 5% or more of retained austenite. However, these conventional technologies have a low yield strength in order to improve the ductility in most cases, which has the disadvantage that the impact resistance is lowered. In addition, considering only ductility, sufficient consideration was not given to bending workability, hole expandability, or weldability required for actual part machining.

Weldability is an essential requirement for high strength steel plates with a yield strength of 750 MPa or more to be used as structural members or impact members. This is because the structural member or the reinforcing member serves to protect the passenger by absorbing the collision energy at the time of collision. If the strength of the weld is insufficient, the structural member or the reinforcement member is broken at the time of collision to obtain sufficient collision absorption energy. Japanese Laid-Open Patent Publication No. 2003-193194 has a problem regarding a high strength steel sheet considering weldability, but there is a problem in that it does not satisfy the weldability required by the market.

In addition, Japanese Laid-Open Patent Publication No. 2005-105367 discloses a technique of securing high yield, weldability and ductility for a high strength steel sheet having a tensile strength of 780 MPa or more. As such, when a high strength steel sheet of 780MPa or more is manufactured in an actual operation process, shape control is difficult due to the high strength of the hot rolled steel sheet, which is an intermediate material, and the cold rolling property is greatly reduced by increasing the rolling load. Because of the need to be applied, there was a problem that the operability such as shape control of the annealing material, surface defects are greatly reduced.

On the other hand, the mini mill process for producing strips by the so-called thin slab continuous casting method, which is a new steel process that has recently attracted attention, can produce a modified tissue steel having a good material deviation because the temperature deviation is small in the longitudinal direction of the strip due to the process characteristics. It is attracting attention as a process with potential. However, most of the well-known patented technologies mainly focus on the cooling technology of hot rolled steel sheet, especially after rolling, and suggest the manufacturing technology of high strength steel having superior material characteristics by using the characteristics of mini mill process. It is not true.

The present invention has been developed in consideration of such a situation in the art, using a thin slab playing method, and through the hot rolling control cooling and cold pressure cold rolling and annealing process without installing a quenching equipment such as water cooling 90 as the final microstructure The main purpose is to provide a method for producing a high strength cold rolled steel sheet having a yield strength of 750 MPa or more, which is excellent in all required properties such as weldability, yield ratio, elongation, bending workability, and the like.

In order to achieve the above object, the present invention provides a manufacturing method as follows.

In the manufacturing method according to the present invention, by weight% C: 0.01 ~ 0.12%, Si: 0.1 ~ 2.0% or less, Mn: 0.5 ~ 2.5%, P: 0.001 ~ 0.10%, S: 0.020% or less, Al: 0.02 ~ 0.30%, N: 0.020% or less, Cr: 0.1 to 1.2%, B: 0.0010 to 0.0080%, tramp element (Cu + Ni + Sn + Pb): 0.18% or less, Ti: 0.003 to 0.08%, Nb: At least one of 0.003% to 0.08% is added and continuously cast steel composed of the remaining Fe and other unavoidable impurities into thin slabs with a thickness of 30 to 150 mm, and the thin slabs are subjected to rough rolling, heating, finishing rolling and winding steps. In the method for producing a hot rolled strip, the hot rolled strip is produced by a pickling, cold rolling, continuous annealing and cold heat treatment step,

The C, Si, Mn, Cr, P, S is [C + Mn / 6 + Cr / 5 ≤ 0.55] (Equation 1) and [C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.27] 2), and the winding step is that the finish rolled strip is cooled to a cooling rate of 20 ℃ / s or more, the winding temperature (CT, unit: ℃) is [555 -440C-14Si-26Mn-11Cr ≤ CT ≤ 600-58C-35Mn-34Cr] (relationship 3).
However, each of the components described in the tramp element, relations 1 to 3 refers to the content (% by weight) of the components.

Another manufacturing method according to the present invention, by weight% C: 0.01 ~ 0.12%, Si: 0.1 ~ 2.0% or less, Mn: 0.5 ~ 2.5%, P: 0.001 ~ 0.10%, S: 0.020% or less, Al: 0.02 ~ 0.30%, N: 0.020% or less, Cr: 0.1 to 1.2%, B: 0.0010 to 0.0080%, tramp element (Cu + Ni + Sn + Pb): 0.18% or less, Ti: 0.003 to 0.08%, Nb : At least one of 0.003 ~ 0.08% is added, and continuous casting of steel composed of the remaining Fe and other unavoidable impurities into thin slabs with a thickness of 30 to 150 mm is carried out, and the thin slabs are subjected to rough rolling, heating, finishing rolling and winding steps. In the method for producing a hot rolled strip, and the hot rolled strip is produced by a pickling, cold rolling, continuous annealing and cold heat treatment step,

The C, Si, Mn, Cr, P, S is [C + Mn / 6 + Cr / 5 ≤ 0.55] (Equation 1) and [C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.27] 2), wherein the cold rolling step consists in cold rolling the pickled strip at a reduction ratio of 10 to 34%.
However, each component described in the above tramp element, relation 1 and relation 2 means the content (% by weight) of the corresponding component.

Another manufacturing method according to the present invention, by weight% C: 0.01 ~ 0.12%, Si: 0.1 ~ 2.0% or less, Mn: 0.5 ~ 2.5%, P: 0.001 ~ 0.10%, S: 0.020% or less, Al: 0.02 to 0.30%, N: 0.020% or less, Cr: 0.1 to 1.2%, B: 0.0010 to 0.0080%, tramp element (Cu + Ni + Sn + Pb): 0.18% or less, Ti: 0.003 to 0.08%, Nb: at least one of 0.003 to 0.08% is added, continuously cast steel composed of the remaining Fe and other unavoidable impurities into a thin slab of 30 to 150 mm thickness, and the thin slab is roughly rolled, heated, finish rolled and wound In manufacturing a hot rolled strip through, the method of producing a high strength cold rolled steel sheet through the pickling, cold rolling, continuous annealing and cold heat treatment step,

The C, Si, Mn, Cr, P, S is [C + Mn / 6 + Cr / 5 ≤ 0.55] (Equation 1) and [C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.27] 2), and the winding step is that the finish rolled strip is cooled to a cooling rate of 20 ℃ / s or more, the winding temperature (CT, unit: ℃) is [555 -440C-14Si-26Mn-11Cr ≤ CT ≤ 600-58C-35Mn-34Cr] (Relationship 3), wherein the cold rolling step consists of cold rolling the pickled strip to a rolling reduction of 10 to 34%.
However, each of the components described in the tramp element, relations 1 to 3 refers to the content (% by weight) of the components.

In the continuous casting step, the casting speed is preferably at least 4.5 mpm, and the rough rolling step is such that the thin slab surface temperature at the entrance of the rough rolling mill is 950 to 1100 ° C., and the cumulative reduction rate at the time of rough rolling is 65. It is preferable to make it to -90%.

In addition, the heating step preferably heats or heats the roughly rolled strip to 920 to 1150 ° C., and the finishing rolling step is such that the rolling speed difference in one strip is 15% or less.

In addition, the continuous annealing step is preferably continuous annealing at a temperature of 740 ~ 820 ℃, the cooling heat treatment step is preferably cooling the continuous annealing strip at a cooling rate of 30 ℃ / s or less.

On the other hand, the low-alloy high strength cold-rolled thin steel sheet excellent in weldability according to the present invention, C: 0.01 ~ 0.12%, Si: 0.1 ~ 2.0% or less, Mn: 0.5 ~ 2.5%, P: 0.001 ~ 0.10%, S : 0.020% or less, Al: 0.02 to 0.30%, N: 0.020% or less, Cr: 0.1 to 1.2%, B: 0.0010 to 0.0080%, tramp element (Cu + Ni + Sn + Pb): 0.18% or less, At least one of Ti: 0.003 to 0.08% and Nb: 0.003 to 0.08% is added, and is composed of the remaining Fe and other unavoidable impurities, wherein C, Si, Mn, Cr, P, and S are [C + Mn / 6 + Cr / 5 ≤ 0.55] (Equation 1) and [C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.27] (Equation 2).
However, each component described in the above tramp element, relation 1 and relation 2 means the content (% by weight) of the corresponding component.

In addition, the cold rolled steel sheet preferably has a ferrite of 95% or more in the final structure.

According to the present invention, through the hot rolled slab playing method and cold rolling and annealing process under light pressure, the yield strength has a high strength of 750MPa or more, even without quenching equipment such as water cooling during annealing, and excellent weldability, elongation rate, yield ratio, bending workability A low alloy high strength thin steel sheet with a small variation can be produced.

The low alloy high strength cold rolled steel sheet manufactured according to the present invention has a high strength of 750MPa and also has a high yield ratio and elongation, and thus can be used as a structural member having excellent impact stability characteristics required in the automobile industry due to its excellent impact energy absorption ability. .

1 is a schematic diagram illustrating a minimill process of the present invention.

Hereinafter, the technical configuration of the present invention will be described in more detail.

As described above, since the present invention relates to a method for producing a low alloy high strength cold rolled thin steel sheet through a mini mill process using a thin slab playing method, first, the mini mill process according to the present invention will be briefly described with reference to FIG.

First, in the continuous casting machine 10 to produce a thin slab (a) having a thickness of 30 ~ 150mm. This is called thin slab in comparison with slabs of 200 mm or more produced by continuous casting machines of conventional mills. Since a slab of 200 mm or more is completely cooled in a yard or the like, it has to be sufficiently reheated to a surface temperature of 1100 ° C. or more in a reheating furnace before hot rolling. On the contrary, since the thin slab is immediately transferred to the roughing mill 20 without passing through the reheating furnace, the heat slab can be used as it is, thereby reducing energy and greatly improving productivity.

After the rolling mill 20 is rolled into a hot rolled strip having a predetermined thickness or less, the temperature of the strip lowered in the process is compensated by the heating means 30, and then the heated hot rolled strip b is finished in the finishing mill 50. Rolled to the desired final thickness, cooled through ROT [Run Out Table 60] (hereinafter referred to as "runout table"), and then finally wound in a constant temperature in the winder 70 to produce a hot rolled steel sheet of the desired material do.

At this time, in order to compensate for the difference between the playing speed and the rolling speed, the coil box 40 may be installed in front of the finish rolling mill 50 so as to be configured to firstly wind the hot rolled strip b passed through the induction heater 30. have. As a high speed playing method of 6mpm or more has been realized recently, a true continuous rolling process without using the coil box 40 has been developed.

The low-alloy high strength cold rolled steel sheet of the present invention manufactured by the above-mentioned mini mill process is C: 0.01 to 0.12%, Si: 0.1 to 2.0% or less, Mn: 0.5 to 2.5%, P: 0.001 to 0.10%, S: 0.020% or less, Al: 0.02 to 0.30%, N: 0.020% or less, Cr: 0.1 to 1.2%, B: 0.0010 to 0.0080%, tramp element (Cu + Ni + Sn + Pb): 0.18% or less At least one of Ti: 0.003 to 0.08% and Nb: 0.003 to 0.08% is added and is composed of the remaining Fe and other unavoidable impurities. The function and content range of each element is briefly described.

As for carbon (C), 0.01 to 0.12 weight% is preferable (All% means weight% unless it demonstrates specially below.). In steel, C combines with carbide-forming elements to deposit carbides or secures strength by strengthening ferrite solid solution, but when it is less than 0.01%, it is difficult to secure strength as a material for automobile parts, and when it exceeds 0.12%, weldability is deteriorated. .

As for silicon (Si), 0.1 to 2.0% or less is preferable. Si in steel suppresses the formation of perlite clusters by improving ferrite strength and inhibiting carbide precipitation by solid solution strengthening. When added in a large amount, the rolling property is very deteriorated and causes surface scale defects. In addition, the content is generally limited to 1.0% or less because it lowers the surface properties of the coated steel sheet and degrades chemical conversion treatment. Due to the advances in the steel content can be produced up to 2.0% without major problems, so the upper limit of the content is limited to 2.0% or less.

Manganese (Mn) is preferably 0.5 to 2.5%. In steel, Mn is an element having a very high solid-solution strengthening effect and promotes the formation of a complex structure composed of ferrite and martensite. If the content is less than 0.5%, while it is difficult to secure the target strength in the present invention, if the content exceeds 2.5% is likely to cause problems such as weldability, hot rolling.

Phosphorus (P) is preferably 0.001 to 0.10%. P in the steel is an element showing the effect of strengthening the steel sheet. If the content is less than 0.001%, not only the effect may not be secured, but it may cause a problem in manufacturing cost, while if the content exceeds 0.10%, press formability may deteriorate and steel brittleness may occur.

Sulfur (S) is preferably 0.020% or less. S in steel is an impurity element that inhibits the ductility and weldability of the steel sheet. If the content exceeds 0.02%, there is a high possibility of inhibiting the ductility and weldability of the steel sheet.

Soluble aluminum (Sol.Al) is preferably 0.02 to 0.3%. Soluble Al in steel removes oxygen present in the steel to prevent formation of non-metallic inclusions during solidification and increases the hardenability by distributing carbon in the ferrite as austenite, such as Si. If the content is less than 0.02%, the effect may not be secured, whereas if the content exceeds 0.3%, the effect may be saturated while the manufacturing cost may increase.

Nitrogen (N) is preferably 0.020% or less. Since N is an impurity element in steel, it is preferable to lower the content as much as possible, so the lower limit has not been set. The upper limit is 0.02% because the performance is inferior and the raw unit is increased when N is excessively added.

Chromium (Cr) is preferably 0.1 to 1.2%. Cr in steel is a hardenable element that prevents surface decarburization and promotes low temperature transformation tissue upon cooling. Precipitates due to precipitate formation elements such as Ti and Nb cause deterioration of hardness of the heat affected zone due to difficulty in reprecipitation after melting due to high temperature rapid heating and rapid cooling during welding, and Cr promotes securing low temperature transformation structure. By compensating for this effect.

In addition, Cr is a component added to improve the hardenability of the steel and to secure high strength, and in the present invention, Cr plays an important role as an element for promoting bainite formation in a hot rolled material state after hot rolling. When the content of Cr is added less than 0.1%, the above-mentioned effect is hardly exhibited, and when the content of Cr is more than 1.2%, the effect is not only saturated but also economically raised by the unit.

As for boron (B), 0.0010 to 0.0080% is preferable. B in steel is a component that delays the transformation of austenite into pearlite during cooling during annealing, and is added as an element that suppresses ferrite formation and promotes the formation of bainite. However, when the content of B is less than 0.0010%, it is difficult to obtain the above effects. When the content of B is more than 0.0080%, grain boundary embrittlement due to grain boundary segregation and excessive B on the surface may be concentrated, resulting in deterioration of plating adhesion.

Meanwhile, in the present invention, one or two selected from titanium (Ti): 0.003 to 0.08% and niobium (Nb): 0.003 to 0.08% may be added to the steel formed as described above to increase the strength and refine the grain size. Can be. When the addition amount of Ti and Nb is less than 0.003%, it is difficult to secure the effect of increasing the strength and miniaturizing the particle size. When the upper limit exceeds 0.08%, the ductility is increased due to the increase in manufacturing cost and excessive precipitates. It can greatly reduce.

The steel tramp element (Cu + Ni + Sn + Pb) is preferably 0.18% or less. The tramp element (Cu + Ni + Sn + Pb) is a kind of impurity element derived from scrap used as a raw material in the steelmaking process, and if its content exceeds 0.18%, it causes the surface crack of thin slab cast slabs. Is limited to 0.18% or less.

The present invention is composed of Fe and other unavoidable impurities in addition to the above components.

According to the present invention, the alloy composition ratio of C, Si, Mn, Cr, P, and S in the alloy design of the thin steel sheet having the above component range preferably satisfies the following components [Relationship 1] and [Relationship 2].

[Relationship 1]

C + Mn / 6 + Cr / 5 ≤ 0.55

[Relation 2]

C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.27

[Equation 1] is obtained from the empirical value of the component relationship capable of ensuring arc weldability quality. That is, the elements of C, Mn, Cr in the steel serves to increase the carbon equivalent during arc welding, the higher the equivalent the deterioration of weldability. When the steel sheet according to the present invention is used as a structural member or a collision member, the condition that welding failure does not occur during arc welding, which is one of the welding methods that are mainly constructed, is set through the repeated experiments as shown in [Relationship 1]. If the value calculated by the relation 1 exceeds 0.55, it means that the possibility of welding failure occurs.

[Equation 2] is obtained by empirical value of the component relationship that can ensure the spot weldability. That is, elements of C, Mn, Si, P, and S in steel serve to increase the carbon equivalent. As is well known, the higher the carbon equivalent, the lower the weldability. When the present invention steel is used by setting the condition that the welding failure does not occur during the spot welding, which is one of the welding methods that are mainly constructed is configured as shown in [Equation 2]. If the value calculated by [Equation 2] exceeds 0.27, it means that the possibility of welding failure occurs.

Hereinafter, a method of manufacturing the steel formed as described above as a cold rolled steel sheet according to the present invention will be described in detail.

The reason for limiting the casting speed to 4.5mpm or more in the continuous casting of the steel composed of the above components with a thin slab of 30 ~ 150mm thickness is as follows. In other words, steel with a yield strength of 750 MPa or more has more elements added for the purpose of securing strength such as C, Mn, and Si in the steel compared to soft products. When the stone is generated, it is difficult to secure the strength and the risk of material deviation is high, so the speed is limited to 4.5mpm or more.

The thin slab continuously cast as described above is immediately rolled in a rough rolling mill of 2 to 4 stands. At this time, it is preferable to make the slab surface temperature at the entrance side of the rough rolling machine to be 950-1,100 ° C. If the surface temperature of the thin slab is less than 950 ℃, the rolling load will not only increase greatly during rough rolling, but also the risk of edge cracking will increase, and if it exceeds 1,100 ℃, the arithmetic scale will occur. It is preferable to limit it to -1,100 degreeC.

In addition, the cumulative reduction rate at the time of rough rolling plays an important role in obtaining a uniform product of the target material in the present invention. In other words, the higher the cumulative reduction rate during rough rolling, the more uniform the microscopic distribution of important elements Mn, Si, Al, etc., and the smaller the temperature gradient in the width and thickness directions of the strip. . However, if the cumulative reduction ratio is less than 65%, the above effects are not sufficiently exhibited. If the cumulative reduction ratio exceeds 90%, the rolling deformation resistance increases and the cost increases significantly, so that the condition becomes 65 to 90%. It is preferable to roll.

On the other hand, the rough-rolled strip is heated and maintained again with a heating device such as an induction heater to maintain a suitable temperature for finishing rolling, when the temperature is less than 920 ℃, the deformation resistance is greatly increased and 1150 ℃ When exceeding, since a scale defect becomes a problem with raise of an energy cost, it is preferable to limit the temperature to 920-1150 degreeC.

In the final rolling of the heated and insulated strip by a tandem rolling mill, it is preferable that the rolling speed difference in one strip is 15% or less. In other words, since the transformation structure of high strength steel of 750MPa grade is used as a reinforcing mechanism in the present invention, the material characteristics are not only largely changed according to the deformation rate during finish rolling, but also the runout table (ROT) is changed when the rolling speed is changed. It is difficult to obtain a uniform cooling rate and target winding temperature in the bed, which in turn causes a large material deviation, so the rolling speed difference is limited to 15% or less.

Finish hot rolling temperature is preferably not less than Ar ₃ transformation point. This is to prevent the two-phase reverse rolling is performed, because in the case of the present invention steel when the two-phase reverse rolling is performed, workability deterioration due to microstructure unevenness is generated by the formation of a mixed structure.

The cooling rate in the ROT after finishing hot rolling was set to 20 ° C./s or more in order to suppress the reduction of hot rolled strength due to the large amount of ferrite transformation and to include the bainite structure.

On the other hand, after finishing hot rolling and ROT cooling, it is preferable to wind up within the winding temperature range determined by following [Relationship Formula 3].

[Relation 3]

555-440C-14 Si  - 26 Mn  -11 Cr  ≤ Coiling temperature ( CT ) ≤ 600-58C-35 Mn  -34 Cr

Winding temperature after finishing hot rolling is the main technical configuration proposed in the present invention, and must be followed to obtain excellent yield strength, elongation, yield ratio, and bending workability in the final thin steel sheet.

[Equation 3] is an empirical formula obtained through a number of experiments, it is proposed that the winding temperature can be the lower bainite inverse. It is confirmed that the configuration of the hot-rolled structure with ferrite and lower bainite is the main technical configuration of the present invention, and the hot-rolled strength and the structure are the main factors affecting the structure and material during cold rolling and annealing performed subsequently.

The hot rolled steel sheet produced through the hot rolling process is subjected to cold rolling after pickling.

The rolling reduction rate in cold rolling is limited to 10 to 34% rather than the usual 40% or more. The high strength steel of yield strength 750MPa or more proposed by the present invention is difficult to control the shape due to the high strength of the hot rolled steel sheet, which is an intermediate material, and the cold rolling property can be greatly reduced by increasing the rolling load, thereby limiting the cold rolling load to limit the cold rolling load. Increasing productivity by reducing also forms a characteristic technical arrangement of the present invention. If the reduction ratio is less than 10% in cold rolling, there is a problem that the recrystallization driving force is weakened to obtain a good recrystallized grain, and if the reduction ratio exceeds 34%, the material is softened and thus the desired strength cannot be obtained.

In the continuous annealing of the cold rolled steel sheet obtained above, the annealing temperature is preferably 740 to 820 ° C. If the temperature during continuous annealing is less than 740 ℃ there is a risk of inferior elongation due to unrecrystallized. When the temperature exceeds 820 ° C., the strength may be degraded due to excessive recrystallization and formation of large grains, and may cause sheet defects such as poor shape due to high temperature annealing operation and surface defects due to surface annealing concentration. In particular, the annealing temperature range is one of the most advantageous effects of the present invention, which generally does not hinder productivity as a temperature for continuously annealing most steel grades.

After continuous annealing, the cooling is slowed down slowly at a cooling rate of 30 ° C / s or less. Since it is cooled by a gentle cooling rate of 30 deg. C / s or less, it is also an advantageous effect of the present invention that no shape distortion due to quenching occurs at all. By continuous annealing under the above conditions, a low alloy high strength thin steel sheet excellent in elongation and weldability of yield strength of 750 MPa or more can be easily produced. If the cooling rate exceeds 30 ℃ / s, not only does it cause a shape defect during cooling, but also inhibits the mail-flowability, and because the quality degradation due to the shape defect of the final product is greatly concerned, cooling at a cooling rate of 30 ℃ / s or less It is preferable.

As described above, it is one of the important technical configurations of the present patent that it is possible to secure a high strength steel having a yield strength of 750 MPa or more even at a relatively slow speed of 30 ° C / s or less in the alloying system proposed in the present invention.

Without undergoing the hot rolling controlled cooling and annealing under annealing proposed in the present invention, an alloying component system satisfying [Relationship 1] and [Relationship 2] simultaneously has a small hardenability, resulting in a relatively soft phase transformation compared to martensite during cooling. It is because yield strength of more than 750MPa cannot be secured without quenching equipment such as water cooling with speed of 80 ~ 100 ℃ / s.

It can be theoretically described as follows that the hot rolling control cooling and the cold rolling under light pressure and annealing proposed in the present invention have excellent characteristics such as high yield strength.

The hot rolled steel structure is composed of ferrite and bainite through cold-rolled cooling to secure proper hot-rolled strength, and increases the strength by hardening under cold rolling under low pressure to secure strength above the target level, and then through continuous annealing. It is to ensure ductility. In this case, it is important to include bainite in the hot rolled structure and to control the reduction rate during cold rolling, and the low rolling rate of bainite decomposition and cold rolling of the hot rolled structure delays recrystallization, thereby enabling continuous annealing under the annealing conditions. .

The steel sheet produced according to the present invention is that the structure of the bainite and martensite is less than 5%, the ferrite is 95% or more, the soft ferrite is composed of 95% or more bendability This is to be improved. The thin steel sheet is used as an automobile steel sheet by being manufactured as a plated steel sheet having a plating layer formed on its surface through a separate plating process.

Hereinafter, the present invention will be described in more detail.

By using the steel composition shown in Table 1 below, the slab thickness, circumferential speed, rolling speed difference, winding upper limit temperature, winding lower limit temperature and winding temperature, cold rolling rate and annealing temperature of Table 2 are annealed under different conditions. After fabricating the plate, the characteristics of each material, that is, tensile strength, elongation, material deviation, bending workability, two-phase tissue fraction, etc. were investigated and shown in Table 2. The chemical composition was expressed in weight%, and the results were calculated and calculated with [Relationship 1] and [Relationship 2], respectively, indicating the arc weldability good range and the spot weldability good range according to the chemical composition. The winding upper limit temperature and the winding lower limit temperature were calculated by [Relationship 3].

[Relationship 1]

C + Mn / 6 + Cr / 5 ≤ 0.55

[Relation 2]

C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.27

[Upper Winding Limit] = 600-58C-35Mn-34Cr

[Lower Winding Limit] = 555-440C-14Si-26Mn-11Cr

Table 2 shows the winding speed, cold rolling temperature, hot rolling temperature, cooling rate after hot rolling, cold rolling rate, annealing temperature, tensile material and final cooling along with winding upper limit and winding lower temperature according to the components based on [Equation 3]. It shows the two-phase fraction (%) of the sheet steel tissue. Yield strength has a high strength of 750 MPa or more, and satisfies the elongation, weldability, and bendability when the above-described manufacturing conditions proposed in the present invention, ie, circumferential speed, rolling speed difference, winding temperature, hot rolling cooling rate, cold rolling rate, and annealing temperature, are satisfied. It is possible to manufacture a high strength low alloy steel sheet.

Comparative Examples 1 and 2 are out of the coiling temperature range while using the invention steel, Comparative Examples 3 to 5 are out of the hot rolling cooling range while using the invention steel, and Comparative Examples 6 to 8 are using the invention steel. It is a case out of the cold rolling reduction range, and Comparative Examples 9 and 10 are cases out of the mini mill process conditions (circumferential conditions, rolling speed difference conditions in the strip) while using the invention steel. On the other hand, Comparative Examples 11-14 are the case manufactured using the comparative steel. According to the above test results, even if the same invention steel is used, the yield strength and tensile strength of the present invention does not comply with the proposed manufacturing conditions, it can be seen that sharply lowered, in particular, the winding temperature range and the cold reduction rate range of the present invention It can be confirmed that the most important manufacturing conditions to achieve the purpose of.

On the other hand, the steel sheet structure of the invention examples can be confirmed that the two-phase fraction is less than 5% ferrite 95% or more.

10: continuous casting machine 20: roughing mill
30: heating means 40: coil box
50: finish rolling mill 60: runout table
70: winder

Claims (11)

By weight% C: 0.01 to 0.12%, Si: 0.1 to 2.0% or less, Mn: 0.5 to 2.5%, P: 0.001 to 0.10%, S: 0.020% or less, Al: 0.02 to 0.30%, N: 0.020% or less , Cr: 0.1 to 1.2%, B: 0.0010 to 0.0080%, Tram element (Cu + Ni + Sn + Pb): 0.18% or less, Ti: 0.003 to 0.08%, Nb: 0.003 to 0.08% Steel, consisting of the remaining Fe and other unavoidable impurities, is continuously cast into thin slabs with a thickness of 30 to 150 mm, and the thin slabs are produced by hot rolling, heating, finishing rolling and winding to produce hot rolled strips. In the method of producing a high strength cold rolled steel sheet through the step of pickling, cold rolling, continuous annealing and cold heat treatment step,
The C, Si, Mn, Cr, P, S is [C + Mn / 6 + Cr / 5 ≤ 0.55] (Equation 1) and [C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.27] 2) configured to meet
The winding step is to cool the finish rolled strip at a cooling rate of 20 ℃ / s or more, the winding temperature (CT, unit: ℃) is [555 -440C-14Si-26Mn-11Cr ≤ CT ≤ 600-58C-35Mn -A method for producing a low alloy high strength cold rolled steel sheet having excellent weldability, characterized by winding within a range that satisfies 34Cr] (Equation 3).
However, each of the components described in the tramp element, relations 1 to 3 refers to the content (% by weight) of the components. delete By weight% C: 0.01 to 0.12%, Si: 0.1 to 2.0% or less, Mn: 0.5 to 2.5%, P: 0.001 to 0.10%, S: 0.020% or less, Al: 0.02 to 0.30%, N: 0.020% or less , Cr: 0.1 to 1.2%, B: 0.0010 to 0.0080%, Tram element (Cu + Ni + Sn + Pb): 0.18% or less, Ti: 0.003 to 0.08%, Nb: 0.003 to 0.08% Steel, consisting of the remaining Fe and other unavoidable impurities, is continuously cast into thin slabs with a thickness of 30 to 150 mm, and the thin slabs are produced by hot rolling, heating, finishing rolling and winding to produce hot rolled strips. In the method of producing a high strength cold rolled steel sheet through the step of pickling, cold rolling, continuous annealing and cold heat treatment step,
The C, Si, Mn, Cr, P, S is [C + Mn / 6 + Cr / 5 ≤ 0.55] (Equation 1) and [C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.27] 2) configured to meet
The winding step is to cool the finish rolled strip at a cooling rate of 20 ℃ / s or more, the winding temperature (CT, unit: ℃) is [555 -440C-14Si-26Mn-11Cr ≤ CT ≤ 600-58C-35Mn -Winding within the range satisfying 34Cr] (Equation 3),
The cold rolling step is a method of manufacturing a low-alloy high strength cold rolled steel sheet excellent in weldability, characterized in that the pickled strip is cold rolled at a reduction ratio of 10 to 34%.
However, each of the components described in the tramp element, relations 1 to 3 refers to the content (% by weight) of the components. The method according to claim 1,
The continuous casting step is a manufacturing method of a low alloy high strength cold rolled steel sheet excellent in weldability, characterized in that the casting speed is 4.5 mpm or more. The method according to claim 1,
In the rough rolling step, the thin slab surface temperature at the entrance side of the rough rolling mill is 950 to 1100 ° C., and the cumulative rolling rate during rough rolling is 65 to 90%. Manufacturing method. The method according to claim 1,
The heating step is a method of manufacturing a low-strength high strength cold rolled steel sheet excellent in weldability, characterized in that to heat or heat the rough-rolled strip to 920 ~ 1150 ℃. The method according to claim 1,
The finishing rolling step is a method of producing a low-alloy high strength cold rolled steel sheet excellent in weldability, characterized in that the rolling speed difference in one strip to 15% or less. The method according to claim 1,
The continuous annealing step is a low alloy high strength cold rolled steel sheet manufacturing method, characterized in that the continuous annealing at a temperature of 740 ~ 820 ℃. The method according to claim 1,
The cooling heat treatment step is a low alloy high strength high strength cold rolled steel sheet manufacturing method, characterized in that for cooling the continuous annealing strip at a cooling rate of less than 30 ℃ / s. delete delete

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